This invention relates to an improvement in anti-skid braking systems for vehicles and in particular to systems in which the braking of the vehicle is effected in a series of cycles of brake application and brake release wherein the start of each brake release and brake application is controlled by a threshold circuit.
The threshold circuit acts upon the braking elements of the wheels through an actuator device which normally consists of a solenoid valve which, when energised, interrupts the existing braking action and which when de-energised re-applies the brakes. Intervention of the threshold circuit is dependent upon a signal caused by the deceleration or acceleration of the wheels and applied to the threshold circuit by a speed differentiator.
The threshold circuit operates within a first threshold level called the "debraking threshold" and a second threshold level called the "re-braking or braking pickup threshold". When, during braking of the vehicle (brake pedal depressed), the deceleration signal provided by the differentiator reaches the first threshold level the threshold circuit trips and causes brake release (debraking) of the wheels by energisation of the solenoid valve. Upon debraking there is an increase in the speed of the vehicle wheels and as soon as the signal from the differentiator reaches the second threshold level, the threshold circuit causes the resumption of braking by de-energisation of the solenoid valve.
In known antiskid braking systems the debraking threshold is set at a constant level and this can bring about a loss of braking efficiency due, for example, to variations in the speed of the vehicle or in the road surface conditions. A low threshold level will be suitable for slow vehicles or for any vehicle at low speed, whilst it is not suitable for fast vehicles. In fact, for fast vehicles, a low braking threshold could result in the intervention of the system (de-braking) due to very small variations of the wheel speed with a consequent series of cycles in which de-braking occurred too quickly. Thus pseudo-skidding conditions could arise to the detriment of braking efficiency, that is, with unduly long stopping distances.
On the other hand a high debraking threshold is suitable for fast vehicles whilst it is not suitable for low speeds or for slow vehicles. In fact, in the latter case a high threshold may give rise to cycles with a tendency for wheel slip, with a consequent danger of wheel locking, resulting in short braking distances, but also in loss of manoevrability of the vehicle.
Moreover, a constant level threshold does not give the best results in the face of road surface variations. If the threshold is high and the vehicle brakes upon ground having a low coefficient of friction, release of the brakes may take place when the wheels have reached high slippage, with a consequent danger of wheel locking. If, on the other hand, the threshold is low, and the vehicle brakes upon ground having a low coefficient of friction, brake release may occur too quickly, before the wheels reach the point of slippage, and the overall braking effect is reduced.